I. Field of the Invention
The present invention relates to digital wireless communications. More particularly, the present invention relates to a novel and improved demodulator for processing a set of user signals that facilitates implementation on a single integrated circuit.
II. Description of the Related Art
FIG. 1 is a block diagram of a highly simplified cellular telephone configured in accordance with the use of a Code Division Multiple Access (CDMA) over-the-air interface. In particular, FIG. 1 illustrates a cellular telephone system configured in accordance with the use of the IS-95 standard, which uses CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. IS-95, and its derivatives such as IS-95A and ANSI J-STD-008 (referred to herein collectively as IS-95), are promulgated by the Telecommunication Industry Association (TIA) as well as other well known standards bodies. Additionally, a cellular telephone system configured substantially in accordance with the use of IS-95 is described in U.S. Pat. No. 5,103,459 entitled xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone Systemxe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same RF band. For example, each mobile unit 10 (typically cellular telephones) shown in FIG. 1 can communicate with a same base station 12 by transmitting a reverse link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station 12 can communicate with mobile units 10 by transmitting a forward link signal over another 1.25 MHz of RF spectrum. Transmitting signals over the same RF spectrum provides various benefits including an increase in the frequency reuse of a cellular telephone system, and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. Soft handoff can be contrasted with hard handoff where the interface with a first base station is terminated before an interface with a second base station is established.
During typical operation of the cellular telephone system of FIG. 1, a base station 12 receives a set of reverse link signals from a set of mobile units 10. The mobile units 10 are conducting telephone calls or other communications. Each reverse link signal is processed within base stations 12, and the resulting data forwarded to base station controller (BSC) 14. BSC 14 provides call resource allocation and mobility management functionality including the orchestration of soft handoffs between base stations. BSC 14 also routes the data received to mobile switching center (MSC), which provides additional routing services for interface with the conventional public switch telephone system (PSTN).
A portion of a prior art base station configured to processing a set of reverse link signals from a set of mobile units 10 is shown in FIG. 2. During operation, antenna system 40 receives a set of reverse link signals transmitted in the same RF band from the set of mobile units 10 in the associated coverage area. RF receiver 42 downconverts and digitizes the set of reverse link signals yielding digital samples that are received by cell site modems (CSMs) 44. Each CSM 44 is allocated by controller 46 to processes a particular reverse link signal from a particular mobile unit 10, and each generates digital data that is forwarded to BSC 14. A system and method for implementing each CSM on a single integrated circuit is described in U.S. Pat. No. 5,654,979 entitled xe2x80x9cCell Site Demodulator Architecture for a Spread Spectrum Multiple Access Communication Systemxe2x80x9d and copending U.S. application Ser. No. 08/316,177 entitled xe2x80x9cMultipath Search Processor For A Spread Spectrum Multiple Access Communication System,xe2x80x9d both assigned to the assignee of the present invention and incorporated herein.
In general, a base station must be capable of interfacing with between sixteen and sixty-four mobile units simultaneously in order to provide adequate capacity for a typical urban appellation. This in turn, requires each base station 12 to contain between 16 and 64 CSMs. While base stations using between 16 and 64 CSMs have been implemented and deployed on a wide scale, the cost of such base stations is relatively high. One of the main causes of this cost is the complex and somewhat sensitive interconnects from the RF unit to the various CSMs, and the interconnects between the base stations controllers and the CSMs. Typically, a subset of twenty-four (24) to thirty-twotwenty-six (3226) or so CSMs are placed on a circuit board, and a set of circuit boards are coupled via a backplane, which in turn is coupled to an RF unit using sets of coaxial cables. Such interconnecting is expensive, and somewhat unreliable, and contributes substantially to the overall cost, complexity and maintenance of a base station 12. Therefore, such a configuration is highly undesirable. The present invention is directed to a method and apparatus for processing a set of reverse link signals received from a set of mobile units without the need for a large set of cell site modems.
FIG. 3 is a block diagram illustrating the signal processing used to transmit a single reverse link traffic channel in accordance with the IS-95 standard provided to facilitate understanding of the invention. Data 48 being transmitted is provided to convolutional encoder 50 in 20 ms segments, called frames, at one of four rates referred to as xe2x80x9cfull ratexe2x80x9d, xe2x80x9chalf ratexe2x80x9d, xe2x80x9cquarter ratexe2x80x9d, and xe2x80x9ceighth ratexe2x80x9d respectively, as each frame contains half as much data as the previous and therefore transmits data at half the rate. Data 48 is typically variable rate vocoded audio information where lower rate frames are used when less information is present, such as during a pause in a conversation. Convolution encoder 50 convolutionally encodes data 48 producing encoded symbols 51, and symbol repeater 52 generates repeated symbols 53 by symbol repeating encoded symbols 51 by an amount sufficient to generate a quantity of data equivalent to a full rate frame. For example, three additional copies of a quarter rate frames are generated for a total of four copies while no additional copies of a full rate frame are generated.
Block interleaver 54 then block interleaves the repeated symbols 53 to generate interleaved symbols 55. Modulator 56 performs 64-ary modulation on interleaved symbols 55 to produce Walsh symbols 57. That is, one of sixty-four possible orthogonal Walsh codes, each code consisting of sixty-four modulation chips, is transmitted for every six interleaved symbols 55. Data burst randomizer 58 performs gating, using frame rate information, on Walsh symbols 57 in pseudorandom bursts such that only one complete instance of the data is transmitted. The gating is performed in increments of six Walsh symbols referred to as xe2x80x9cpower control groups,xe2x80x9d because a power control command is generated at the base station every corresponding period. Sixteen power control groups occur for each 20 ms frame, with all sixteen being transmitted for a full rate frame, eight for a half rate frame, four for a quarter rate frame and two for an eighth rate frame. For each lower rate frame the power control groups transmitted are a subset of the groups transmitted for a higher rate frame.
The gated Walsh chips are then direct sequence modulated using a pseudorandom (PN) long channel code 59 at rate of four long channel code chips to each Walsh chip generating modulated data 61. The long channel code is unique for each mobile unit 10 and is known by each base station 12. Modulated data 61 is duplicated with the first copy being xe2x80x9cspreadxe2x80x9d via modulation with an in-phase pseudorandom spreading code (PNI) producing I-channel data, and the second copy is delayed one half a spreading code chip by delay 60 and spread via modulation with a quadrature-phase pseudorandom spreading code (PNQ) producing Q-channel data. The PNI and PNQ spread data sets are each low pass filtered (not shown), before being used to modulate in-phase and quadrature-phase carrier signals respectively. The modulated in-phase and quadrature-phase carrier signals are summed together before transmitted to a base station or other receive system (not shown).
The present invention is a novel and improved system and method for performing the digital receive processing for multiple signals received over the same RF band. In a preferred embodiment of the invention, digital RF samples are stored in a RAM queue which is accessed by a searcher and a demodulator. The searcher and demodulator are preferably located on the same integrated circuit along with the RAM queue. The demodulator demodulates a set of reverse link signals stored within the RAM queue where each reverse link signal is received at a particular time offset and processed using a particular channel code. The searcher periodically searches for reverse link signals not being processed by the demodulator, and for access requests transmitted via the access channel. The searcher preferably searches during the worthy power control groups of each reverse link signal, which corresponds to the two of sixteen power control groups transmitted during a eighth rate frame.